Glycol system (secondary loop)
Also known as: secondary loop, secondary refrigerant system, brine system
Short definitionAn indirect cooling arrangement: the refrigerant circulates only in the chiller in the plant room, and pumped glycol–water carries the cooling to the coolers in the rooms. Refrigerant charge and the chance of a leak drop sharply.
The chiller cools a mixture of glycol and water, and a pump sends this liquid through pipes to the room coolers and process points. The liquid does not evaporate there; it warms up and comes back. Because the refrigerant circuit stays short, the charge is 70–90% smaller than in a direct expansion installation. Ammonia or propane never enters the area where the food is. Adding a room later only means extending the pipework.
Non-toxic propylene glycol is chosen for food work; ethylene glycol carries heat better but is used elsewhere. The mixing ratio is set so that the liquid will not freeze even 5–8 K below the lowest evaporating temperature; more than that loads the pump and weakens heat transfer. Because of the extra heat exchanger, evaporation is kept 4–6 K colder and total electricity usually rises by 10–20%. Ready-mixed inhibited glycol is used, and galvanised pipe is never laid.